1. Field of the Invention
The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue and, more particularly, to a radio frequency (xe2x80x9cRFxe2x80x9d) ablation apparatus for controlling the flow of current through biological tissue so that the depth and continuity of ablation lesions may be controlled.
2. Description of the Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cSA nodexe2x80x9d) located in the wall of the right atrium. The SA node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cAV nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth, remodeling, or damage to the conductive tissue in the heart can interfere with the passage of regular electrical signals from the SA and AV nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed percutaneously, i.e., a procedure in which a catheter is introduced into the patient through an artery or vein and directed to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves the formation of continuous atrial incisions that prevent atrial reentry and allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. In ablation therapy, transmural lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. In this sense transmural is meant to include lesions that pass through the atrial wall or ventricle wall from the interior surface (endocardium) to the exterior surface (epicardium).
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In both the unipolar and the bipolar methods, the current traveling between the electrodes of the catheter and between the electrodes and the backplate enters the tissue and induces a temperature rise in the tissue resulting in the creation of ablation lesions.
During ablation, RF energy is applied to the electrodes to raise the temperature of the target tissue to a lethal, non-viable state. In general, the lethal temperature boundary between viable and non-viable tissue is between approximately 45xc2x0 C. to 55xc2x0 C. and more specifically, approximately 48xc2x0 C. Tissue heated to a temperature above 48xc2x0 C. for several seconds becomes permanently non-viable and defines the ablation volume. Tissue adjacent to the electrodes delivering RF energy is heated by resistive heating which is conducted radially outward from the electrode-tissue interface. The goal is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C. In clinical applications, the target temperature is set below 70xc2x0 C. to avoid coagulum formation. Lesion size has been demonstrated to be proportional to temperature.
A basic RF ablation system for forming linear lesions includes a catheter carrying a plurality of electrodes, a backplate and an RF generator adapted to provide RF signals to the electrodes to establish bipolar or unipolar current flow. In one such ablation system, as described in U.S. Pat. No. 6,200,314, RF signals having a fixed frequency and amplitude and a controllable phase angle are supplied to each electrode. A backplate is maintained at a reference voltage level in relation to the amplitude of the RF signals. The power control system controls the relative phase angles of the RF signals to establish a voltage potential between the electrodes. Current thus flows between the electrodes and between the electrodes and the backplate to produce linear lesions. In order to establish the phase difference between RF signals, the system requires a programmable logic array and a controllable frequency source. The logic array receives phase control signals from a microprocessor and controls the frequency source accordingly.
In other less complex RF ablation systems, such as those described in U.S. Pat. Nos. 5,810,802 and 6,001,093, a controller electrically couples an indifferent electrode, i.e., backplate, and each of several electrodes to a single RF source through a network of switches. Depending on the setting of its associated switch, an electrode may be set to either an energy emitting polarity, an energy receiving polarity or neither (inactive). Using the switches, the system may be configured so that current flows between the electrodes or between the electrodes and the backplate. The system, however, does not provide for simultaneous unipolar and bipolar operation, thus lesion depth and continuity characteristics may be inadequate.
Hence, those skilled in the art have recognized a need for a multi-channel ablation system having power output control capability for providing periodically fluctuating voltage potentials between electrodes to thereby induce unipolar and bipolar current flow through tissue without reliance on complex phasing circuitry. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to multi-channel ablation systems having controllable power output capability for providing periodically varying voltage potentials between electrodes to thereby establish unipolar or bipolar current flow through biological tissue.
In one aspect, the invention relates to an ablation system including a catheter having a plurality of electrodes and a power generator. The power generator provides a first power output having a first frequency to at least one electrode defining a first electrode set and a second power output having a second frequency to at least one electrode defining a second electrode set. The first frequency is different then the second frequency. The frequency difference establishes a voltage potential between the electrode sets resulting in bipolar current flow between the electrodes.
In a detailed aspect, the first frequency and the second frequency are such that the voltage difference of the first power output with respect to the second power output yields a resultant waveform having a period no greater than approximately 100 microseconds, to thereby prevent stimulation of excitable muscular and cardiac tissue. In further detailed aspects, exemplary second-frequency-to-first-frequency ratios and resultant waveform periods include 3:1 and 2 microseconds, 2:1 and 2 microseconds, 1.5:1 and 4 microseconds, 1.25:1 and 8 microseconds and 0.5:1 and 4 microseconds. In another detailed facet of the invention, the ablation system includes a processor that periodically switches the first and second power outputs such that during a first time period the first power output is provided to the first electrode set and the second power output is provided to the second electrode set and during a second time period the first power output is provided to the second electrode set and the second power output is provided to the first electrode set.
In another aspect, the invention relates to a system for delivering energy to biological tissue associated with a biological site. The system includes a catheter carrying an electrode system having a plurality of electrodes at its distal end. The electrode system is adapted to be positioned proximal to the biological tissue. The system further includes a backplate that is adapted to be positioned proximal to the biological site so that the biological tissue is interposed between the electrode system and the backplate. Further included in the system is a power control system that provides a first power output having a first frequency to at least one electrode defining a first electrode set and a second power output having a second frequency, different then the first frequency, to at least one electrode defining a second electrode set. The frequency difference provides bipolar current flow between the electrodes. The power control system also establishes a voltage potential between the backplate and at least one of the first and second electrode sets, thereby providing unipolar current flow.
In another facet, the invention relates to a power control system for delivering energy to biological tissue interposed between a plurality of electrodes. The power control system includes a power generator that provides a first power output having a first frequency and a second power output having a second frequency, different then the first frequency. The power control system also includes a processor programmed to control the power generator such that the first power output is provided to at least one electrode defining a first electrode set and the second power output is provided to at least one electrode defining a second electrode set.
In a detailed aspect, the power generator is adapted to provide the first and second power outputs from a single frequency source. In a further detailed aspect, the frequency source provides a power output waveform and the power generator includes at least two frequency dividers. Each divider is adapted to receive the power output waveform and provide the first power output and second power output, respectively. In a still further detailed aspect, the dividers are adapted to provide the first power output and the second power output such that the voltage difference of the first power output with respect to the second power output yields a resultant waveform having a period no greater than approximately 100 microseconds.
In another aspect, the invention is related to a method of delivering energy to biological tissue associated with a biological site. The method includes positioning a catheter having a plurality of electrodes proximal to the biological tissue and providing a first power output having a first frequency to at least one electrode defining a first electrode set and a second power output having a second frequency, different then the first frequency, to at least one electrode defining a second electrode set.
In a detailed facet, the method further includes selecting the first frequency and the second frequency such that the voltage difference of the first power output with respect to the second power output yields a resultant waveform having a period no greater than approximately 100 microseconds. In another detailed aspect, the method also includes periodically switching the first and second power outputs such that during a first time period the first power output is provided to the first electrode set and the second power output is provided to the second electrode set and during a second time period the first power output is provided to the second electrode set and the second power output is provided to the first electrode set. In a further detailed aspect, each of the first and second power outputs has a duty cycle with alternating on and off periods and the periodic switching occurs during the off portion of the duty cycle. In another further detailed aspect, the first frequency and the second frequency are such that the voltage difference of the first power output with respect to the second power output yields a resultant waveform having a period no greater than approximately 100 microseconds and the switching occurs at a time substantially equal to a multiple of the period of the resultant waveform.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.